1. Field of the Invention
This invention relates to free radical polymerization processes employing cobalt(II) chelates of vicinal iminohydroxyimino compounds, dihydroxyimino compounds, diazadihydroxyiminodialkyldecadienes and diazadihydroxyiminodialkylundecadienes as catalytic chain transfer agents for controlling the molecular weight of the homopolymers and copolymers produced.
2. Background
In any polymerization process it is necessary to be able to control the molecular weight of the polymer produced so that it may be fitted to a particular use or need. For example, in unperturbed polymerization systems which fundamentally tend to produce high molecular weight polymers, it may be desirable or necessary to limit the molecular weight of the polymers produced, and this must be done in a predictable and controllable fashion. Such molecular weight limitation may be desirable or necessary in the preparation of polymer solutions for use in paints and finishes which require high solids contents to assure reduced solvent emission during application, and yet which require low viscosity to facilitate ready application.
In free radical polymerizations there are several conventional means of effecting such molecular weight limitation, but all have notable disadvantages. These include:
(1) The use of a high initiator/monomer ratio, but this is costly in terms of initiator consumption. PA1 (2) Polymerizing at high temperatures, for example, about 150.degree. C., which is undesirably energy intensive. PA1 (3) Adding stoichiometric amounts of thiol chain transfer agents to the polymerizing system, but the attendant incorporation of sulfur-containing agents into the polymer renders it less durable than is desired. PA1 (4) Odor problems associated with the use of sulfur-containing chain transfer agents.
Catalytic chain transfer to the monomer as a means of controlling molecular weight in the radical polymerization of methyl methacrylate and styrene in the presence of cobalt(II) porphyrin complexes is known in the art. N. S. Enikolopyan et al., J. Polym. Sci., Polym. Chem. Ed., Vol. 19, 879 (1981), describe the kinetics and the molecular weight control achieved in the free radical polymerization of methyl methacrylate in the presence of a cobalt complex of hematoporphyrin tetramethyl ether. This use of this cobalt complex is also discussed by B. R. Smirnov et al. in Vysokomol. soyed., A23, No. 5, 1042 (1981) and by B. R. Smirnov et al. in Dokl. Akad. Nauk SSSR, 253, 891 (1980). In a similar study, B. R. Smirnov et al., Dokl. Akad. Nauk SSSR, 254, 127 (1980), describe studies carried out with hematoporphyrin tetramethyl ester. The authors conclude that only the combination of cobalt with a tetrapyrrole porphyrin ligand apparently permits the realization of catalysis of chain transfer to the monomer, and that it has been ruled out that analogous phenomena will be discovered during the investigation of complexes similar in spatial and electronic structure to the porphyrins, such as phthalocyanines, corrins, cobaloximes, etc. B. R. Smirnov et al., Vysokomol. soyed., A23, No. 11, 2588 (1981), describe the catalytic chain transfer observed in the radical polymerization of styrene in the presence of a cobalt complex of hematoporphyrin IX tetramethyl ester.
D. E. Pashchenko et al., Dokl. Akad. Nauk SSSR, 265, 889 (1982), describe chain transfer studies with cobalt porphyrins in the polymerization of methyl methacrylate. Regarding this paper, it is not understood what is meant by "cobalt complexes of porphyrins and cobaloximes" since there is no further mention of "cobaloximes" in the English language version of the paper; moreover, the term "cobaloximes" does not appear in the original Russian text, but rather the term "cobalamines", which are vitamin B12-related structures similar to the porphyrin structures disclosed in this paper and in the other publications cited above.
Although the use of the porphyrin complexes circumvents many of the problems associated with the aforesaid conventional commercial processes, the complexes impart too much color to the final product, rendering it useless or less desirable in many applications, such as in certain paints and finishes. Moreover, the cost of the porphyrin complex is rather high.
A. F. Burczyk et al., J. Polym. Sci., Polym. Chem. Ed., Vol. 22, 3255 (1984), disclose that cobaloximes, that is, bisdimethylgloximatocobalt complexes, are often used as analogs of cobalt porphyrins in research studies, and they further disclose the use of cobaloxime, synthesized from Co(II) acetate and dimethylglyoxime, as a relatively cheap chain transfer agent in the free radical polymerization of methyl methacrylate. The cobaloxime of Burczyk et al. is shown on page 3256 as being of the formula ##STR1## wherein B is a coordinating base ligand, such as triphenylphosphine. Similar disclosures are made by A. F. Burczyk in a thesis to the University of Waterloo, Waterloo, Ontario, 1984.
Carlson et al., U.S. Pat. No. 4,526,945, also disclose the use of cobalt(II) dimethylgloxime and similar cobalt(II) dioxime pyridine complexes such as those derived from 2,3-butanedione, 2,3-hexanedione, 2,4-heptanedione, 2,5-dimethyl-3,4-hexanedione, 3-methyl-2,4-hexanedione, 1,2-cyclohexanedione, 3-phenyl-2,4-pentanedione, 2-naphthylglyoxal, camphoroquinone, 4-chloro-1,2-benzoquinone, 1,2-napthoquinone, 3,7-dimethyl-1,2-napthoquinone, 3-chloro-1,2-napthoquinone or substituted 1,2-anthraquinones as molecular weight control agents in homogeneous polymerizations.
H. C. Rai et al., Indian Journal of Chemistry, Vol. 18A, 242 (1979), describe the preparation of the cobalt chelates of cobalt(II) with 4,7-diaza-2,9-dihydroxyimino-3,8-dimethyldeca-3,7-diene and cobalt(II) with 4,8-diaza-2,1-dihydroxyimino-3,9-dimethylundeca-3,8-diene. There is no disclosure as to the use of these compounds as catalytic chain transfer agents. E. Uhlig et al., Z. anorg. allg. Chem., 343, 299 (1966), describe the preparation of the ligands 4,7-diaza-2,9-dihydroxyimino-3,8-dimethyldeca-3,7-diene and 4,8-diaza-2,10-dihydroxyimino-3,9-dimethylundeca-3,8-diene compounds. G. N. Schrauzer, Inorg. Syn., 11, 62, 64 (1968), describes the preparation of diaqua bis(2,3-dihydroxyiminobutanato)Co(II). A. Bakac et al., J. Am. Chem. Soc., 106, 5197 (1984), describe the preparation of Co(II)(2,3-dioxyiminobutane-BF.sub.2).sub.2 (H.sub.2 O).sub.2.
It is an object of this invention to provide cobalt(II) chelates of vicinal iminohydroxyimino compounds, dihydroxyimino compounds, diazadihydroxyiminodialkyldecadienes and diazadihydroxyiminodialkylundecadienes which operate as highly efficient catalytic chain transfer agents for controlling homopolymer and copolymer molecular weights, while imparting very little, if any, color to the final products, making them more useful in many applications, such as in paints and finishes.
Another object is to provide such a process which proceeds at modest temperatures, for example, about 65.degree. to about 110.degree. C., with limited amounts of initiator and in the absence of stoichiometrically-reacting chain transfer agents. These and other objects will become apparent hereinafter.